On Sun, 20 Oct 2002, Andrew Morton wrote:
> Well I'm assuming that you don't want to sleep if, say,
> ep->eventcnt is non-zero. The code is currently (simplified):
>
> add_wait_queue(...);
> if (ep->eventcnt)
> break;
> /* window here */
> set_current_state(TASK_INTERRUPTIBLE);
> schedule();
>
> If another CPU increments eventcnt and sends this task a wakeup in that
> window, it is missed and we still sleep. The conventional fix for that
> is:
>
> add_wait_queue(...);
> set_current_state(TASK_INTERRUPTIBLE);
> if (ep->eventcnt)
> break;
> /* harmless window here */
> schedule();
>
> So if someone delivers a wakeup in the "harmless window" then this task
> still calls schedule(), but the wakeup has turned the state from
> TASK_INTERRUPTIBLE into TASK_RUNNING, so the schedule() doesn't actually
> take this task off the runqueue. This task will zoom straight through the
> schedule() and will then loop back and notice the incremented ep->eventcnt.
>
> So it is important that the waker increment eventcnt _before_ delivering
> the wake_up, too.
It's true ... but the window is pretty small there :) Anyway it makes the
code more correct and I changed it. I have the new patch with your
suggestions ready and I will post as sonn as it'll pass a few tests.
- Davide
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This archive was generated by hypermail 2b29 : Wed Oct 23 2002 - 22:00:51 EST